1. Field of the Invention
The present invention relates to a system for determining a position of a mobile object such as an autonomous robot, automobile, etc., and more particularly to a learning type self-position determining device for autonomously determining a position at which the mobile object itself exists in a mobile space by learning.
2. Description of the Prior Art
How to determine the position of a mobile object has been studied as a fundamental technology for achieving autonomy of a motion of a robot, automobile, etc. Normally, the position of a mobile object is calculated according to the information about a motion of the mobile object or the information about the relative position of a clue which helps to locate the absolute position. The following document 1 refers to this technology, which is outlined as follows.
Document 1: Edited by Japanese Robot Society: Robot Engineering Handbook Published by Corona Co. Ltd.
The first technology is a method called dead reckoning, which infers the relative position of a mobile object according to the information about the contact between the mobile object and the surface on which the mobile object moves. A straight move of a mobile object is measured by using a rotary encoder on the wheels, a wheel speed sensor, odometer, etc., while a change of a move direction of the mobile object is measured by performing rotation calculation of a differential wheel, or by using a compass, etc. Accumulation of such measurement results allows the distance between the mobile object and the origin and the direction of the mobile object from the origin to be obtained.
The second technology is a method using inertial navigation. With this method, both of an accelerometer and an angular velocity sensor which can measure a motion in an inertial coordinate system, are used in order to infer a position. The position can be inferred with high accuracy by using a gyro compass which can accurately find the absolute azimuth instead of the angular velocity sensor.
The third technology is a method using triangulation in a beacon system. With this method, the position of a mobile object in a coordinate system of landmarks is calculated according to the distance from each of the landmarks arranged in a move space to the mobile object, the relative angle between the landmarks within which the mobile object is centered, and the position information of a landmark.
The above described three methods have various advantages and disadvantages depending on the information used for determining a position. For the present invention, how to determine a position is explained by classifying the information for determining a position into return information and sense information. The return information is information about a motion of a mobile object. It can be obtained by processing a signal of a sensor which measures a motion state of the mobile object. The sense information is information about an environment of the mobile object. It can be obtained by, for example, processing a signal of a sensor which measures the distance between the mobile object and a landmark or the relative angle between the landmarks, or of a sensor which detects the absolute azimuth.
The dead reckoning as the first method is a method using the return information. Its advantages are that system cost is low, and an error when a position is determined becomes smaller if a motion path is in good condition in which slippage, etc. does not occur. Its disadvantage is that correction must be made depending on a road state because errors due to slippage of a wheel are accumulated.
According to the inertial navigation as the second method, there is the case in which only the return information is used, that is, the case in which the relative azimuth is detected using a mechanical gyro, and the case in which both of the return information and sense information are used, that is, the case in which the absolute azimuth is detected using a gyro compass.
The advantage of the first case in which only the return information is used is that errors due to slippage between a wheel of a mobile object and a surface on which the wheel contacts can be prevented from being accumulated. This is because the return information is generated by measuring a motion state using an angular velocity sensor and an accelerometer according to the inertial navigation. The disadvantage is that slight errors of signals of a sensor which measures a motion state are accumulated, and especially, the accuracy of the angle information is a problem. Use of an optical fiber gyro rather than a mechanical gyro provides a higher accuracy and its size is smaller.
The advantages when both the return and sense information are used are that errors due to slippage between a wheel of a mobile object and a contact surface can be prevented from being accumulated, and accurate angle information can be obtained using the absolute azimuth, thereby significantly improving the performance of determining a position. The disadvantages are that an extremely precise device, which is expensive, is required, a time constant of a response is normally long, and this implementation is unsuitable for a system of a small size.
The method using triangulation as the third method employs the sense information. Its advantage is that errors are not accumulated. Its disadvantage is that the information of an environment such as a map, beacon, landmark (a map which clearly shows the position information of such landmarks) must be provided when a system is designed, and the accuracy of the position determination is determined depending on the precision of the map.
Any of the above described methods cannot be selected and used alone when the position of an autonomous robot is to be determined. The first and second methods basically integrate a path using the return information. Accordingly, if a mobile object is carried to an arbitrary point not using a proper move method, for example, if an automobile is carried to an arbitrary point by a trailer, the position information is lost and its position cannot be determined. When the method for integrating a path is used, a straight line distance to the origin and its azimuth can be maintained as the information as long as the path is continuous. However, if the path becomes discontinuous, such information cannot be maintained.
Even if a gyro compass can be used for the second method, the position information cannot be modified because the absolute azimuth is isotropic information at each point. That is, if the north direction is indicated as the absolute azimuth, the position information cannot be modified only with that information. Similarly, the accumulation of an error included in a signal in a motion state cannot be modified.
For the triangulation method using the sense information as the third method, a system designer or a user must provide the map information of an environment in which a robot is positioned, in advance. Additionally, if the environment is dynamically changed, the map information must be modified. This means that the position cannot be autonomously determined, and even that the position cannot be identified at all in the state in which a landmark cannot be detected.